1. Field of the Invention
The present invention relates to a method for simultaneously removing a plurality of different heavy metal ions from contaminated water. More particularly, the present invention is concerned with a method in which a plurality of different heavy metal ions, such as a nickel ion and a cobalt ion, are efficiently removed from contaminated water, for example, water to be used for and waste water from a nuclear power generator. The present invention is also concerned with a heavy metal ion-adsorptive, porous membrane, which is especially useful for simultaneous removal of a plurality of different heavy metal ions from contaminated water.
2. Discussion of Related Art
Conventionally, an ion exchange resin has been used for removing a plurality of different heavy metal ions from contaminated water, for example, water to be used for and waste water from a nuclear power generator. The conventionally employed ion exchange resin is used in the form of a spherical gel having a relatively large particle size, e.g., having a particle diameter of tens of micrometers. When contaminated water is passed through the gels, the ions present in the contaminated water are adsorbed on the gels due to the diffusion (equilibrium) of the ions between the stream of contaminated water and the interior portions of the gels. The efficiency of adsorption of the ions on the gel depends on the balance of the diffusion rate of the ions into the gels and the diffusion rate of the ions into the stream of contaminated water. In this connection, it is noted that since the particle size of the gel of ion exchange resin is relatively large, the gaps between the gels are also large. Therefore, the ions are likely to pass through the gaps so that the adsorption efficiency is lowered. Accordingly, for sufficiently removing the ions from contaminated water by adsorption on the gels, it is necessary that the contaminated water be passed through a large quantity of gels. When contaminated water contains a plurality of different heavy metal ions, complicated reactions, namely, adsorption and desorption of each of the different ions, are caused to occur during the passage of the contaminated water through the gels due to the competition between the ions having different adsorption equilibriums for the gel. Therefore, the efficiency of adsorption is much lower when the removal of different heavy metal ions from contaminated water is intended. For sufficiently removing different heavy metal ions from contaminated water, it is necessary to use an extremely large quantity of ion exchange resin. Further, the flow rate of contaminated water passing through the gels must be strictly controlled for pre-venting the leakage of ions. Moreover, complicated, delicate change of operation is needed according to the variation in quality of contaminated water, such as variations of the types and amounts of coexisting ions. Therefore, the operation is extremely troublesome.